The present invention relates generally to the manufacture of semiconductor devices and, in particular, to methods for fabricating such devices using anti-reflective layers, as well as devices including anti-reflective coatings.
The fabrication of integrated circuits requires the precise positioning of a number of regions in a semi-conductor wafer, followed by one or more interconnection patterns. The regions include a variety of implants and diffusions, cuts for gates and metallizations, and windows in protective cover layers through which connections can be made to bonding pads. A sequence of steps is required for each such region.
Photolithographic techniques, for example, can be used in the performance of some or all of the foregoing operations. Typically, for example, the surface of a wafer to be processed is pre-coated with a photoresist. The photoresist then is exposed to a light source with a suitably patterned mask positioned over the wafer. The exposed resist pattern is used, for example, to open windows in a protective underlying layer to define semiconductor regions or to delineate an interconnection pattern.
To improve the degree of integration and to obtain high density devices, performing photolithographic operations at shorter wavelengths is desirable. Currently, i-line techniques with a wavelength of about 365 nanometers (nm), KrF excimer laser techniques with a wavelength of about 248 nm, and KnF excimer laser techniques with a wavelength of about 193 nm are used. However, at those wavelengths, optical reflections at the interfaces of previously-formed layers on the semiconductor wafer can cause notching of the photoresist.
FIG. 1 illustrates the general nature of the problem. A semiconductor wafer 10 includes one or more previously-formed layers 12 covered by a thick layer of boro-phospho-silicate glass (BPSG) 14. The BPSG layer 14 serves as a protective layer for the underlying layers 12 and also provides a more planar surface. A photoresist film 16 is coated over the BPSG layer 14, and a mask 18 is positioned over the photoresist prior to exposure of the photoresist to an appropriate source of radiation 20. The mask 18 can be used to define, for example, contact holes for one of the previously-formed layers 12.
Ideally, when the photoresist film 16 is exposed to the radiation 20, the mask 18 precisely defines the dimensions of the exposed regions of the photoresist film. However, the BPSG layer 14 is transparent to the wavelengths typically used in photolithography, including 248 nm and 365 nm. Thus, a significant amount of the radiation 20 that passes through the mask 18 travels through the BPSG layer 14 and is reflected at the interface between the BPSG layer and one or more of the previously-formed underlying layers 12. Some of the reflected radiation (indicated by arrow 22) contributes to exposure of the photoresist film 16.
In some situations, a dielectric anti-reflective coating is provided above the BPSG layer to reduce reflections from the underlying layers. However, if the structures in the previously-formed underlying layers 12 have varying dimensions or varying shapes and the level of reflected light is relatively high, the reflected light 22 will expose the photoresist film 16 non-uniformly leading to the formation of notching.
In general, techniques are disclosed for fabricating a device using a photolithographic process. The techniques are particularly advantageous for transferring an optical pattern by photolithography to one or more layers which are transparent to the wavelength(s) at which the photolithography is performed.
According to one aspect, a method includes providing a first anti-reflective coating over a surface of a substrate. As used herein, the term xe2x80x9csubstratexe2x80x9d refers to one or more semiconductor layers or structures which may include active or operable portions of semiconductor devices. Various films or other materials may be present on the semiconductor layers or structures. A layer which is transparent to a wavelength of light used during the photolithographic process is provided over the first anti-reflective coating, and a photosensitive material is provided above the transparent layer. The photosensitive material is exposed to a source of radiation including the wavelength of light. Preferably, the first anti-reflective coating extends beneath substantially the entire transparent layer.
According to another aspect, a semiconductor device includes a layer that is transparent to light having a wavelength, for example, of approximately 193 nm, 248 nm or 365 nm. A first anti-reflective coating extends substantially entirely beneath the transparent layer.
One advantage of providing an anti-reflective coating beneath the transparent layer is that the anti-reflective coating can help reduce notching of the photosensitive material that may occur during the photolithographic process.
In general, the complex refractive index of the first anti-reflective coating can be selected to maximize (or increase) the absorption at the first anti-reflective coating to minimize (or reduce) the amount of light transmitted through the first anti-reflective coating and reflected back from the underlying structures. Therefore, the effects of the non-uniform structures in the layers below the first anti-reflective coating can be reduced or eliminated. That, in turn reduces the amount of light that is reflected back toward the photosensitive material and, therefore, further reduces notching.
Various implementations include one or more of the following features. The transparent layer can include a material such as BPSG, PSG and TEOS. Other materials, including various oxides and nitrides, also can be used as the transparent layer. Depending on the properties of the photosensitive material, it can be exposed to radiation at various wavelengths including approximately 193 nm, 248 nm or 365 nm. Portions of the photosensitive material selectively can be exposed to the radiation.
The first anti-reflective coating can include various materials, including a material comprising silicon and nitrogen; silicon and oxygen; or silicon, oxygen and nitrogen. Other materials, such as organic polymers, also can be used as the first anti-reflective coating.
According to another aspect, in addition to the first anti-reflective coating, a second anti-reflective coating can be provided above the transparent layer. The photosensitive material then can be provided over the second anti-reflective coating. Providing the second anti-reflective coating between the photosensitive material and the transparent layer can further help reduce the effects of light that is reflected from the interface of the first anti-reflective coating and the transparent layer.